Why do reactions either give out or absorb heat?
3 marks · NormalDescribe, using bond breaking and bond making, how you can tell if a chemical reaction will be exothermic or endothermic.
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If more energy is released when forming new bonds than is required to break the original bonds, the reaction is exothermic. If more energy is required to break bonds than is released in making bonds, the reaction is endothermic.
Also accept: Exothermic: energy released > energy absorbed. Endothermic: energy absorbed > energy released.
Common mistakes: Confusing which process absorbs/releases energy.
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Marking points
B1 · 1 mark · States that bond breaking requires energy; bond making releases energy.
A1 · 1 mark · Links exothermic to energy released when making bonds > energy needed to break bonds.
A2 · 1 mark · Links endothermic to energy required to break bonds > energy released by making bonds.
Worked solution
To tell if a reaction is exothermic or endothermic, compare the energy needed to break bonds in the reactants with the energy released making bonds in the products. If more energy is released when forming bonds than is needed to break reactant bonds, the reaction is exothermic. If it is the other way round, it is endothermic.